What exactly is an external enclosure needed for?

I am a newbie about to upgrade my laptop hard drive (80GB to 320GB) and have read a lot about using an external enclosure to house the old hard drive. I have searched (for "external enclosure") and read dozens of posts but none really explain what this thing does, when it is needed etc.

So I am wondering what exactly are they used for? Is it mainly a way to just transfer the files from old HD to new HD? Or do people keep it hooked up to their laptop and use it as an extra hard drive? I guess I'm wondering b/c I have an external backup HD for my desktop that I can use to transfer, so I'm wondering if I need an enclosure.

Sorry if this is a dumb question but like I said I searched and couldn't find an answer.

Thanks in advance,

 

It's for protecting the hard drive as well as giving you a way to hook it up to a computer via another interface, such as USB or Firewire. It also provides power to the drive either through USB or through it's own AC adapter. Some people use this as a method to transfer data, as well as long term use. It's 80GB, might as well do something with it.

 

It is basically a way to use spare hard drives on a laptop with 1 hdd bay. There are three ways to hook up an external hdd. USB, Firewire and ESATA. E-SATA is the fastest of them all, then firewire then usb. The desktop drive you have I am guessing is 3.5" so you would have to get a 3.5" external enclosure. So basically the idea is to have a way to copy all old files from old hdd to new hdd, and have a backup.

 

Minor correction, most Firewire ports are IEEE 1394 400. 400mbps. Only a few are 800mbps. USB 2.0 is 480mbps.
Not to mention drives never reach these throughputs anyway.

 

OK, so it seems the consensus is that this is a good thing to have, especially since it's only $10 or so.

My Dell 1505 takes a 2.5", and I'm getting the below HD..

http://www.newegg.com/Product/Product.aspx?Item=N82E16822145228

so I guess I look for an enclosure of that size.

Thanks all.